1. Field of the Invention
This invention relates to a light scanning system, and more particularly to a light scanning system in which a semiconductor laser is employed as a light source.
2. Description of the Related Art
A semiconductor laser has been in wide use as a light source in various light scanning systems such as a recording system, a reading system and the like because of its various advantages. For example, the semiconductor laser is small in size, low in cost and small in power consumption as compared with a gas laser and permits direct analog modulation, that is, the output of the semiconductor laser can be changed by controlling the drive current.
Such a light scanning system generally comprises a semiconductor laser, a scanning optical system including a polygonal mirror, an f.theta. lens and the like which condenses a laser beam emitted from the semiconductor laser and causes the laser beam to scan a predetermined surface, and a surface tilt compensating optical system such as a cylindrical mirror or a cylindrical lens which is disposed between the semiconductor laser and the scanning optical system and corrects fluctuation in position of scanning spots due to a surface tilt in a deflector such as the polygonal mirror.
The semiconductor laser is very small (in the order of 1 .mu.m) in width of an active layer (core) which emits a laser beam and is in the order of the oscillating wavelength in size of the opening in the output window. Accordingly the divergence angle of the laser beam emitted through the opening is extremely larger than that in the gas laser and the beam divergence angle itself fluctuates by about .+-.10% from laser to laser.
For example, in a GaAlAs laser diode, though the standard value of the beam divergence angle (at the full width at half maximum) in a direction perpendicular to the direction of junction is 38.degree. under a predetermined condition, the beam divergence angle can fluctuate from 20.degree. (smaller than the standard value by 18.degree.) to 45.degree. (larger than the standard value by 7.degree.).
The laser beam output with such a beam divergence angle is generally collimated into parallel rays by a collimator lens or the like and the collimated laser beam is converged to a desired beam diameter and caused to scan a predetermined surface at the beam waist thereof by a condensing/scanning optical system, whereby reading or recording of information is effected.
Since the intensity of the scanning laser beam exhibits Gaussian distribution, the beam divergence angle 2.theta. of the laser beam, the wavelength .lambda. of the laser beam and the beam diameter r0 at the beam waist satisfy the following formula (1). EQU r0=.lambda..PI..theta. (1)
As can be understood from the formula (1), the beam diameter r0 at the beam waist becomes smaller as the beam divergence angle .theta. increases.
Since the beam divergence angle of a laser beam as emitted from a semiconductor laser fluctuates from laser to laser as described above, the spot size of the laser beam scanning the surface differs by the semiconductor laser used, which results in fluctuation, for instance, in image quality of the recorded image such as resolution from system to system.
It is preferred that the spot size be constant irrespective of the semiconductor laser used.
There has been proposed, as disclosed for instance in Japanese Unexamined Patent Publication No. 63(1988)-51687, a technique for condensing spontaneous emission which is emitted from a semiconductor laser together with a laser beam into a spot as small as that of the laser beam. When a mask plate having an opening which permits only light in the central portion of the laser beam to pass therethrough is disposed in the path of the laser beam emitted from the semiconductor laser in accordance with the technique, light in the periphery of the laser beam is cut and the spot size on the surface to be scanned can be constant irrespective of the beam divergence angle of the laser beam as emitted from the semiconductor laser, whereby fluctuation in image quality can be prevented.
However this approach is disadvantageous in that a part of the laser beam is cut and lost by the mask plate and accordingly efficiency of utilization of the laser beam is deteriorated.